A. Field of the Invention
This invention relates to microcapsules and to a process for their production. In particular, this invention relates to encapsulated droplets of a liquid material which is substantially insoluble in water, where the encapsulating agent is a film formed from a modified urea-formaldehyde polymer.
B. Description of the Prior Art
The use of membranes, coatings, and capsules for the controlled release of liquid materials is well known in the art of both agricultural and non agricultural chemicals. In agriculture, controlled-release techniques have improved the efficiency of herbicides, insecticides, fungicides, bactericides, and fertilizers. Non-agricultural uses include encapsulated dyes, inks, pharmaceuticals, flavoring agents, and fragrances.
The most common forms of controlled-release materials are coated droplets or microcapsules, coated solids including both porous and non-porous particles, and coated aggregates of solid particles. In some instances, a water-soluble encapsulating film is desired, which releases the encapsulated material when the capsule is placed in contact with water. Other coatings are designed to release the entrapped material when the coating is ruptured by external force.
Still further coatings are porous in nature and release the entrapped material to the surrounding medium at a slow rate by diffusion through the pores. In addition to providing controlled release, such coatings also serve to facilitate the dispersion of water-immiscible liquids into water and water-containing media such as wet soil. Droplets encapsulated in this manner are particularly useful in agriculture, where water from irrigation, rain, and water sprays is frequently present. A variety of processes for producing such capsules is known.
In one process, the capsules are formed by phase separation from an aqueous solution through the coacervation of a hydrophilic colloid sol. This is described in U.S. Pat. Nos. 2,800,457 (Green et al., July 23, 1957) and 2,800,458 (Green, July 23, 1957).
An interfacial polymerization process is disclosed in U.S. Pat. Nos. 4,046,741 (Scher, Sept. 6, 1977) and 4,140,516 (Scher, Feb. 20, 1979), whereby the film-forming reactants are dissolved in the hydrophobic liquid which is dispersed in water, the reaction occurring at the interface when the phases are placed in contact as an emulsion.
A further interfacial polymerization process is described in U.S. Pat. No. 3,726,804 (Matsukawa et al., Apr. 10, 1973) whereby all the film-forming ingredients initially reside in hydrophobic droplets which also contain a low boiling or polar solvent in addition to the material to be encapsulated. Upon heating, the solvent is released into the aqueous phase (the continuous phase of the emulsion), and the film-forming materials accumulate at the interface and polymerize.
Olefin polymerization using a peroxide catalyst is described in Japanese patent publication No. 9168/1961, whereby an oil-insoluble polymer is formed at the surfaces of oil drops.
British Patent Nos. 952,807 and 965,074 describe a process whereby a solid such as wax or a thermoplastic resin is melted, dispersed and cooled to form an encapsulating film around liquid droplets.
U.S. Pat. No. 3,111,407 (Lindquist et al., Nov. 19, 1963) describes a spray drying method which forms encapsulated droplets at the instant of atomization.
These processes vary in terms of equipment expense, energy requirements, ease of controlling the microcapsule size, the need for extra reagents such as catalysts and settling agents, and percent microcapsule phase. It is therefore an object of the present invention to provide a simple, inexpensive method for producing microcapsules of uniform and readily controlled size, which are suitable for use without further treatment. Other objects of the invention will be apparent from the following description.